Ultrasonic imaging applications are being driven to higher frequencies to enable the imaging of smaller and smaller anatomical features. Very high frequency ultrasonic imaging systems, critical for improved diagnostic applications in dermatology and ophthalmology, await the development of arrays operating above 20 MHz. These high frequency arrays require small spatial scales (&lt;10 .mu.m) which cannot be achieved using conventional fabrication techniques.
Prior art arrays developed in this frequency range include a pair of 20 MHz PZT arrays. See: M. Lethiecq, et al., "Miniature High Frequency Array Transducers Based on New Fine Grain Ceramics," 1994 Ultrasonics Symposium, 1994, pp. 1009-1013; and A. Nguyen-Dinh et al., "High Frequency Piezo-Composite Transducer Array Designed for Ultrasound Scanning Applications," 1996 IEEE Ultrasonics Symposium, 1996, pp. 943-947. Ito et al. describe a 100 MHz array incorporating a sapphire lens and thin film ZnO in "A 100 MHz Ultrasonic Transducer Array Using ZnO Thin Films, "IEEE Transactions on UFFC", vol. 42, no. 2, pp. 316-324, March 1995. Payne. et al. describe a piezoelectric polymer array with built-in transmit and receive circuitry. See: "Integrated Ultrasound Traducers," 1994 Ultrasonics Symposium, 1994, pp. 1523-1526.
O'Donnel et al describe the operation of a 20 MHz phased array imaging system for catheter use. See: M. O'Donnel and L. J. Thomas, "Efficient Synthetic Aperture Imaging from a Circular Aperture with Possible Applications to Catheter Based Imaging, "IEEE Transactions on UFFC, vol. 39, no. 3, pp. 366-380, May 1992.
Linear arrays, which typically do not use beam-steering, can tolerate a much larger element pitch than phased arrays. It is thus practical to focus first on the development of linear arrays with 1.lambda. to 2.lambda. pitch. Although integrating such an array with the electronics offers advantages, a more conventional and flexible approach of coupling the array elements to a 50 ohm imaging system has been adopted. Broad bandwidth (minimum of 40%) is desired, both to suppress grating lobes and to improve the axial resolution. Crosstalk levels of near -30 dB are considered acceptable for a linear array not incorporating Doppler. Finally, mild elevational focusing is desired for improved resolution in the elevation direction.
PZT "strip" vibrators (length&gt;&gt;width or height) with free boundary conditions require a width to height ratio of less than approximately 0.6 for proper pulse performance. For 30 MHz operation each PZT strip must therefore measure approximately 30 .mu.m wide by 50 .mu.m in height. For a 2-2 composite comprised of interleaved polymer and ceramic plates, optimized operation requires that the spatial scale of all constituents be much less than a wavelength. For example, the epoxy between each ceramic strip must be less than 10 .mu.m in order to push spurious lateral resonances above the passband of a 30 MHz array. Conventional dice and fill technology cannot presently be used to manufacture this structure.
An alternative fabrication technique is to stack ceramic and polymer or other inert layers to form a block of composite material, then slice sections from this block. See U.S. Pat. Nos. 4,514,247 and 4,572,981 to Zola. Variants of this technique have been proposed for arrays operating above 10 MHz. J. Stevenson et al. in, "Fabrication and Characterization of PZT/Thermoplastic Polymer Composites for High Frequency Phased Arrays," J. Am. Ceramic Soc., 77[9], pp. 2481-2484, 1994, describe a method for fabricating a PZT/polymer composite transducer. Their method involves the bonding of sheets of thermoplastic polymer film and sintered PZT plates via thermal processing. The composites are then cut, electroded and poled to produce the required transducer structure.
The difficulty with the Stevenson et al. process is in controlling the ceramic and polymer dimensions. Without precise dimensional control, transducer operations are not reproducibly predictable.
Accordingly, it is an object of the invention to provide a method for the manufacture of high frequency ultrasound transducers which assures precise dimensional control of the ultimate transducer structure.
It is another object of this invention to provide a method of manufacture of piezoelectric composites and ultrasonic transducers incorporating such composites that are operable at frequencies in a range of 20 MHz and higher.